This invention relates to improvements in nozzles or ducts embracing a screw propeller of large diameter such that there is a significant differential between the hydrostatic pressure imposed upon the tip of a propeller blade when in the position of greatest submergence vertically below the propeller boss and the pressure imposed upon the tip of the same blade when in the position of minimum submergence vertically above the propeller boss. The invention provides means for substantially reducing this pressure differential and equalise the loading on the blade roots around the propeller, reducing vibration and noise.